How to change a specific colour in an image
When separating colors, it is separating the primary colors.
This means red's hex will be: FF0000
Blue's hex will be: 0000FF
Green's hex will be: 00FF00  

This means if you have a hex of 00FBFF and separate the colors, and only keep green, the 00FBFF will become green, because it's still got the G in it.



So when trying to change the color blue, you can't change the color blue and then combine it back with the other colors because it won't be blue. It won't function properly because you connected a different color with blue. 

When separating RGBA, instead of changing Blue into BW, and then using color balance, create another Combine RGBA node and plug the Blue from the Separate RGBA into the B input (To create a proper input for B). Next, create a Hue/Saturation node, and plug the new Combine RGBA into the Hue/Saturation node. Finally, add a Mix RGB node and plug both Combine RGBA nodes into each input. Change the Mix RGB node into an Add RGB node, and plug the Add RGB node into the Viewer and/or Composite.

Now if you change the hue in the Hue/Saturation node, the blue color alone should change. 



This is because we separated blue from the rest of the image, and then changed it, and replaced the color blue with something else. It works because it is separate until after the combine RBGA node.

Before


After (I change blue to the color yellow because why not
)

I hope I explained this well, and I hope I helped.